How to drill 4 mm holes in 1 mm. Steel and avoid grates?

Stepped bits have also worked for me for drilling into thin sheet metal - round holes every time versus jagged holes with regular bits.

I have a set of stepped bits that start at 3mm diameter.
 

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If you're gonna drill: start with a 3mm (1/8") bit. Use cutting oil, 3-in-1, something. After making the first hole with a twist bit, get a Step Bit to make the larger holes. A twist bit will bend thin sheet metal as it goes in, don't use big twist bits on thin sheet metal. There are many deburring tools out there, for sheet metal I like to use a Countersink Bit, but not one with flutes, they chatter. Use one with a hole through it, like this:

View attachment 1317143


This will give you the smoothest edge.

Try to practice a couple cuts on similar material, if possible. Go slow. Have fun!

w
Thanks. What deburring countersink bit SIZE would you recommend for a 4 mm. hole ?
 
That many holes?
Sorry but not doable by hand, even with a bench drill, because 4mm twist drill is too thin and flexes, so you will lose alignment
Proper solution is punching them.

Plan B is to cut a window the size of the whole ventilation area, say 5 by 10 cm or whatever you need, it will be easier to get clean edges or worst case file them down, and cut a proper sized rectangle out of commercial perforated sheet (which is punched) and resistance spot solder or worst case Epoxy it in place.

Spray painting everything in semi-gloss black paint will disguise any small imperfections.

We are talking home accessible solutions here, the chassis you show was CNC punched before final bending and painting.
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Plan B is to cut a window the size of the whole ventilation area, say 5 by 10 cm or whatever you need, it will be easier to get clean edges or worst case file them down, and cut a proper sized rectangle out of commercial perforated sheet (which is punched) and resistance spot solder or worst case Epoxy it in place.
I call these inlays. Another reason to just DIY the whole case in the first place with a basic desktop CNC
 
Plan B is to cut a window the size of the whole ventilation area, say 5 by 10 cm or whatever you need, it will be easier to get clean edges or worst case file them down, and cut a proper sized rectangle out of commercial perforated sheet (which is punched) and resistance spot solder or worst case Epoxy it in place.
Pretty good plan in my view. You can also attach it with screws. It can be made to look nice.

Tom